From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp4.osuosl.org (smtp4.osuosl.org [IPv6:2605:bc80:3010::137]) by lists.linuxfoundation.org (Postfix) with ESMTP id A5388C000F for ; Mon, 20 Sep 2021 15:10:34 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp4.osuosl.org (Postfix) with ESMTP id 7E17F403F3 for ; Mon, 20 Sep 2021 15:10:34 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -2.098 X-Spam-Level: X-Spam-Status: No, score=-2.098 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no Authentication-Results: smtp4.osuosl.org (amavisd-new); dkim=pass (2048-bit key) header.d=gmail.com Received: from smtp4.osuosl.org ([127.0.0.1]) by localhost (smtp4.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id z3NT9jZu1VCj for ; Mon, 20 Sep 2021 15:10:30 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.8.0 Received: from mail-qk1-x732.google.com (mail-qk1-x732.google.com [IPv6:2607:f8b0:4864:20::732]) by smtp4.osuosl.org (Postfix) with ESMTPS id 5E6B740325 for ; Mon, 20 Sep 2021 15:10:30 +0000 (UTC) Received: by mail-qk1-x732.google.com with SMTP id c7so38249814qka.2 for ; Mon, 20 Sep 2021 08:10:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=UQhswQ1aQlNZ5FN40M4+INKUCYXpsXjLXJ1P3r1fh40=; b=R0Yayg1ZOkgkxFymFqNcszXwur9mbUgX8qyUlEFkl2H7s8HSFROWqXRImAv2/REW3r gTqtlxDw8MvitUjJWufPne/UU5+gAhmt4+9A6ppPBzY6hIdFcouWmXyB+U3oSVV5Q5nf GeHKZUpJB1a9vv9RFUBstHATJ4yiZnnTecuFwrGU6s/FNDMOnzPo3AwrYY9qjnwOqrmY PqJSEebcABaRtN1oRaMDh4Vmg4CZjE4zQTS31kqJncrEDWa8iA4GBSNP1o/tsM42YE/7 S+ePwrYLaFXzO8XEp0usME6K+pMfIfkNzuK63tNGbF4sw9kkriCo3qsCy69ol3lWRpev KObA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=UQhswQ1aQlNZ5FN40M4+INKUCYXpsXjLXJ1P3r1fh40=; b=WoceVQc8RW2wFGGdpKBrSCxGfgHRKtmMJo1Y5k2NjIbjM36ptW14ii0PoMLiWlO4Tv KDGlcqy8UOk2Wx1tkujUydHm3gdH8k6jEd0wZEeccyWrqwNDWrdq+BDMYjdO16r6xUsT 3BYgGvCQ3i8YB41sWbAgVZjOmPd0bJhzVVvuK954B11C6doyvmPS0fbnWUJ9Lzifn3Qf mhCjxmHyanrZsHST4kg27mVeNZzV1CupPXUKEoZZmYN1chOpbNTgREsan4DS5gKk6eVb VFgcST4DEnLXYRXp+OAv6EGxtqaTNyhByPJIBGE+F7FzvXbfrYrHKXF7GOxi/bHi9ox/ GI/g== X-Gm-Message-State: AOAM530auzszsfbOG+G9nY7vVvzBbWlffizb6in3C7dQeDyiKZgRsQri v/2hHfYV9PcJbAiJg6sbBxzk/3oiM32YCYUrP8VGCwD2LRY= X-Google-Smtp-Source: ABdhPJyEpup22cEbL9DmKI4ymte9XZXxrfKaExoMwZgUNnz3ONqL4gFPn0W1KXmNP8egmonbnscodQt8/XaQHoMIgJc= X-Received: by 2002:a25:5cb:: with SMTP id 194mr23196109ybf.408.1632150625026; Mon, 20 Sep 2021 08:10:25 -0700 (PDT) MIME-Version: 1.0 References: In-Reply-To: From: Gloria Zhao Date: Mon, 20 Sep 2021 16:10:14 +0100 Message-ID: To: Antoine Riard Content-Type: multipart/alternative; boundary="0000000000003984f005cc6eae1f" X-Mailman-Approved-At: Mon, 20 Sep 2021 15:52:27 +0000 Cc: Bitcoin Protocol Discussion Subject: Re: [bitcoin-dev] Proposal: Package Mempool Accept and Package RBF X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 20 Sep 2021 15:10:34 -0000 --0000000000003984f005cc6eae1f Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi Antoine, First of all, thank you for the thorough review. I appreciate your insight on LN requirements. > IIUC, you have a package A+B+C submitted for acceptance and A is already in your mempool. You trim out A from the package and then evaluate B+C. > I think this might be an issue if A is the higher-fee element of the ABC package. B+C package fees might be under the mempool min fee and will be rejected, potentially breaking the acceptance expectations of the package issuer ? Correct, if B+C is too low feerate to be accepted, we will reject it. I prefer this because it is incentive compatible: A can be mined by itself, so there's no reason to prefer A+B+C instead of A. As another way of looking at this, consider the case where we do accept A+B+C and it sits at the "bottom" of our mempool. If our mempool reaches capacity, we evict the lowest descendant feerate transactions, which are B+C in this case. This gives us the same resulting mempool, with A and not B+C. > Further, I think the dedup should be done on wtxid, as you might have multiple valid witnesses. Though with varying vsizes and as such offering different feerates. I agree that variations of the same package with different witnesses is a case that must be handled. I consider witness replacement to be a project that can be done in parallel to package mempool acceptance because being able to accept packages does not worsen the problem of a same-txid-different-witness "pinning" attack. If or when we have witness replacement, the logic is: if the individual transaction is enough to replace the mempool one, the replacement will happen during the preceding individual transaction acceptance, and deduplication logic will work. Otherwise, we will try to deduplicate by wtxid, see that we need a package witness replacement, and use the package feerate to evaluate whether this is economically rational. See the #22290 "handle package transactions already in mempool" commit ( https://github.com/bitcoin/bitcoin/pull/22290/commits/fea75a2237b46cf761452= 42fecad7e274bfcb5ff), which handles the case of same-txid-different-witness by simply using the transaction in the mempool for now, with TODOs for what I just described. > I'm not clearly understanding the accepted topologies. By "parent and child to share a parent", do you mean the set of transactions A, B, C, where B is spending A and C is spending A and B would be correct ? Yes, that is what I meant. Yes, that would a valid package under these rules. > If yes, is there a width-limit introduced or we fallback on MAX_PACKAGE_COUNT=3D25 ? No, there is no limit on connectivity other than "child with all unconfirmed parents." We will enforce MAX_PACKAGE_COUNT=3D25 and child's in-mempool + in-package ancestor limits. > Considering the current Core's mempool acceptance rules, I think CPFP batching is unsafe for LN time-sensitive closure. A malicious tx-relay jamming successful on one channel commitment transaction would contamine the remaining commitments sharing the same package. > E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitment transactions and E a shared CPFP. If a malicious A' transaction has a better feerate than A, the whole package acceptance will fail. Even if A' confirms in the following block, the propagation and confirmation of B+C+D have been delayed. This could carry on a loss of funds. Please note that A may replace A' even if A' has higher fees than A individually, because the proposed package RBF utilizes the fees and size of the entire package. This just requires E to pay enough fees, although this can be pretty high if there are also potential B' and C' competing commitment transactions that we don't know about. > IMHO, I'm leaning towards deploying during a first phase 1-parent/1-child. I think it's the most conservative step still improving second-layer safety. So far, my understanding is that multi-parent-1-child is desired for batched fee-bumping ( https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-897951289) and I've also seen your response which I have less context on ( https://github.com/bitcoin/bitcoin/pull/22674#issuecomment-900352202). That being said, I am happy to create a new proposal for 1 parent + 1 child (which would be slightly simpler) and plan for moving to multi-parent-1-child later if that is preferred. I am very interested in hearing feedback on that approach. > If A+B is submitted to replace A', where A pays 0 sats, B pays 200 sats and A' pays 100 sats. If we apply the individual RBF on A, A+B acceptance fails. For this reason I think the individual RBF should be bypassed and only the package RBF apply ? I think there is a misunderstanding here - let me describe what I'm proposing we'd do in this situation: we'll try individual submission for A, see that it fails due to "insufficient fees." Then, we'll try package validation for A+B and use package RBF. If A+B pays enough, it can still replace A'. If A fails for a bad signature, we won't look at B or A+B. Does this meet your expectations? > What problem are you trying to solve by the package feerate *after* dedup rule ? > My understanding is that an in-package transaction might be already in the mempool. Therefore, to compute a correct RBF penalty replacement, the vsize of this transaction could be discarded lowering the cost of package RBF. I'm proposing that, when a transaction has already been submitted to mempool, we would ignore both its fees and vsize when calculating package feerate. In example G2, we shouldn't count M1 fees after its submission to mempool, since M1's fees have already been used to pay for its individual bandwidth, and it shouldn't be used again to pay for P2 and P3's bandwidth. We also shouldn't count its vsize, since it has already been paid for. > I think this is a footgunish API, as if a package issuer send the multiple-parent-one-child package A,B,C,D where D is the child of A,B,C. Then try to broadcast the higher-feerate C'+D' package, it should be rejected. So it's breaking the naive broadcaster assumption that a higher-feerate/higher-fee package always replaces ? Note that, if C' conflicts with C, it also conflicts with D, since D is a descendant of C and would thus need to be evicted along with it. Implicitly, D' would not be in conflict with D. More generally, this example is surprising to me because I didn't think packages would be used to fee-bump replaceable transactions. Do we want the child to be able to replace mempool transactions as well? This can be implemented with a bit of additional logic. > I think this is unsafe for L2s if counterparties have malleability of the child transaction. They can block your package replacement by opting-out from RBF signaling. IIRC, LN's "anchor output" presents such an ability. I'm not sure what you mean? Let's say we have a package of parent A + child B, where A is supposed to replace a mempool transaction A'. Are you saying that counterparties are able to malleate the package child B, or a child of A'? If they can malleate a child of A', that shouldn't matter as long as A' is signaling replacement. This would be handled identically with full RBF and what Core currently implements. > I think this is an issue brought by the trimming during the dedup phase. If we preserve the package integrity, only re-using the tx-level checks results of already in-mempool transactions to gain in CPU time we won't have this issue. Package childs can add unconfirmed inputs as long as they're in-package, the bip125 rule2 is only evaluated against parents ? Sorry, I don't understand what you mean by "preserve the package integrity?" Could you elaborate? > Let's say you have in-mempool A, B where A pays 10 sat/vb for 100 vbytes and B pays 10 sat/vb for 100 vbytes. You have the candidate replacement D spending both A and C where D pays 15sat/vb for 100 vbytes and C pays 1 sat/vb for 1000 vbytes. > Package A + B ancestor score is 10 sat/vb. > D has a higher feerate/absolute fee than B. > Package A + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats + C's 1000 sats + D's 1500 sats) / A's 100 vb + C's 1000 vb + D's 100 vb) I am in agreement with your calculations but unsure if we disagree on the expected outcome. Yes, B has an ancestor score of 10sat/vb and D has an ancestor score of ~2.9sat/vb. Since D's ancestor score is lower than B's, it fails the proposed package RBF Rule #2, so this package would be rejected. Does this meet your expectations? Thank you for linking to projects that might be interested in package relay :) Thanks, Gloria On Mon, Sep 20, 2021 at 12:16 AM Antoine Riard wrote: > Hi Gloria, > > > A package may contain transactions that are already in the mempool. We > > remove > > ("deduplicate") those transactions from the package for the purposes of > > package > > mempool acceptance. If a package is empty after deduplication, we do > > nothing. > > IIUC, you have a package A+B+C submitted for acceptance and A is already > in your mempool. You trim out A from the package and then evaluate B+C. > > I think this might be an issue if A is the higher-fee element of the ABC > package. B+C package fees might be under the mempool min fee and will be > rejected, potentially breaking the acceptance expectations of the package > issuer ? > > Further, I think the dedup should be done on wtxid, as you might have > multiple valid witnesses. Though with varying vsizes and as such offering > different feerates. > > E.g you're going to evaluate the package A+B and A' is already in your > mempool with a bigger valid witness. You trim A based on txid, then you > evaluate A'+B, which fails the fee checks. However, evaluating A+B would > have been a success. > > AFAICT, the dedup rationale would be to save on CPU time/IO disk, to avoi= d > repeated signatures verification and parent UTXOs fetches ? Can we achiev= e > the same goal by bypassing tx-level checks for already-in txn while > conserving the package integrity for package-level checks ? > > > Note that it's possible for the parents to be > > indirect > > descendants/ancestors of one another, or for parent and child to share = a > > parent, > > so we cannot make any other topology assumptions. > > I'm not clearly understanding the accepted topologies. By "parent and > child to share a parent", do you mean the set of transactions A, B, C, > where B is spending A and C is spending A and B would be correct ? > > If yes, is there a width-limit introduced or we fallback on > MAX_PACKAGE_COUNT=3D25 ? > > IIRC, one rationale to come with this topology limitation was to lower th= e > DoS risks when potentially deploying p2p packages. > > Considering the current Core's mempool acceptance rules, I think CPFP > batching is unsafe for LN time-sensitive closure. A malicious tx-relay > jamming successful on one channel commitment transaction would contamine > the remaining commitments sharing the same package. > > E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitment > transactions and E a shared CPFP. If a malicious A' transaction has a > better feerate than A, the whole package acceptance will fail. Even if A' > confirms in the following block, > the propagation and confirmation of B+C+D have been delayed. This could > carry on a loss of funds. > > That said, if you're broadcasting commitment transactions without > time-sensitive HTLC outputs, I think the batching is effectively a fee > saving as you don't have to duplicate the CPFP. > > IMHO, I'm leaning towards deploying during a first phase 1-parent/1-child= . > I think it's the most conservative step still improving second-layer safe= ty. > > > *Rationale*: It would be incorrect to use the fees of transactions tha= t > are > > already in the mempool, as we do not want a transaction's fees to be > > double-counted for both its individual RBF and package RBF. > > I'm unsure about the logical order of the checks proposed. > > If A+B is submitted to replace A', where A pays 0 sats, B pays 200 sats > and A' pays 100 sats. If we apply the individual RBF on A, A+B acceptance > fails. For this reason I think the individual RBF should be bypassed and > only the package RBF apply ? > > Note this situation is plausible, with current LN design, your > counterparty can have a commitment transaction with a better fee just by > selecting a higher `dust_limit_satoshis` than yours. > > > Examples F and G [14] show the same package, but P1 is submitted > > individually before > > the package in example G. In example F, we can see that the 300vB packa= ge > > pays > > an additional 200sat in fees, which is not enough to pay for its own > > bandwidth > > (BIP125#4). In example G, we can see that P1 pays enough to replace M1, > but > > using P1's fees again during package submission would make it look like= a > > 300sat > > increase for a 200vB package. Even including its fees and size would no= t > be > > sufficient in this example, since the 300sat looks like enough for the > 300vB > > package. The calculcation after deduplication is 100sat increase for a > > package > > of size 200vB, which correctly fails BIP125#4. Assume all transactions > have > > a > > size of 100vB. > > What problem are you trying to solve by the package feerate *after* dedup > rule ? > > My understanding is that an in-package transaction might be already in th= e > mempool. Therefore, to compute a correct RBF penalty replacement, the vsi= ze > of this transaction could be discarded lowering the cost of package RBF. > > If we keep a "safe" dedup mechanism (see my point above), I think this > discount is justified, as the validation cost of node operators is paid f= or > ? > > > The child cannot replace mempool transactions. > > Let's say you issue package A+B, then package C+B', where B' is a child o= f > both A and C. This rule fails the acceptance of C+B' ? > > I think this is a footgunish API, as if a package issuer send the > multiple-parent-one-child package A,B,C,D where D is the child of A,B,C. > Then try to broadcast the higher-feerate C'+D' package, it should be > rejected. So it's breaking the naive broadcaster assumption that a > higher-feerate/higher-fee package always replaces ? And it might be unsaf= e > in protocols where states are symmetric. E.g a malicious counterparty > broadcasts first S+A, then you honestly broadcast S+B, where B pays bette= r > fees. > > > All mempool transactions to be replaced must signal replaceability. > > I think this is unsafe for L2s if counterparties have malleability of the > child transaction. They can block your package replacement by opting-out > from RBF signaling. IIRC, LN's "anchor output" presents such an ability. > > I think it's better to either fix inherited signaling or move towards > full-rbf. > > > if a package parent has already been submitted, it would > > look > >like the child is spending a "new" unconfirmed input. > > I think this is an issue brought by the trimming during the dedup phase. > If we preserve the package integrity, only re-using the tx-level checks > results of already in-mempool transactions to gain in CPU time we won't > have this issue. Package childs can add unconfirmed inputs as long as > they're in-package, the bip125 rule2 is only evaluated against parents ? > > > However, we still achieve the same goal of requiring the > > replacement > > transactions to have a ancestor score at least as high as the original > > ones. > > I'm not sure if this holds... > > Let's say you have in-mempool A, B where A pays 10 sat/vb for 100 vbytes > and B pays 10 sat/vb for 100 vbytes. You have the candidate replacement D > spending both A and C where D pays 15sat/vb for 100 vbytes and C pays 1 > sat/vb for 1000 vbytes. > > Package A + B ancestor score is 10 sat/vb. > > D has a higher feerate/absolute fee than B. > > Package A + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats + C's 1000 > sats + D's 1500 sats) / > A's 100 vb + C's 1000 vb + D's 100 vb) > > Overall, this is a review through the lenses of LN requirements. I think > other L2 protocols/applications > could be candidates to using package accept/relay such as: > * https://github.com/lightninglabs/pool > * https://github.com/discreetlogcontracts/dlcspecs > * https://github.com/bitcoin-teleport/teleport-transactions/ > * https://github.com/sapio-lang/sapio > * https://github.com/commerceblock/mercury/blob/master/doc/statechains.md > * https://github.com/revault/practical-revault > > Thanks for rolling forward the ball on this subject. > > Antoine > > Le jeu. 16 sept. 2021 =C3=A0 03:55, Gloria Zhao via bitcoin-dev < > bitcoin-dev@lists.linuxfoundation.org> a =C3=A9crit : > >> Hi there, >> >> I'm writing to propose a set of mempool policy changes to enable package >> validation (in preparation for package relay) in Bitcoin Core. These >> would not >> be consensus or P2P protocol changes. However, since mempool policy >> significantly affects transaction propagation, I believe this is relevan= t >> for >> the mailing list. >> >> My proposal enables packages consisting of multiple parents and 1 child. >> If you >> develop software that relies on specific transaction relay assumptions >> and/or >> are interested in using package relay in the future, I'm very interested >> to hear >> your feedback on the utility or restrictiveness of these package policie= s >> for >> your use cases. >> >> A draft implementation of this proposal can be found in [Bitcoin Core >> PR#22290][1]. >> >> An illustrated version of this post can be found at >> https://gist.github.com/glozow/dc4e9d5c5b14ade7cdfac40f43adb18a. >> I have also linked the images below. >> >> ## Background >> >> Feel free to skip this section if you are already familiar with mempool >> policy >> and package relay terminology. >> >> ### Terminology Clarifications >> >> * Package =3D an ordered list of related transactions, representable by = a >> Directed >> Acyclic Graph. >> * Package Feerate =3D the total modified fees divided by the total virtu= al >> size of >> all transactions in the package. >> - Modified fees =3D a transaction's base fees + fee delta applied by >> the user >> with `prioritisetransaction`. As such, we expect this to vary acro= ss >> mempools. >> - Virtual Size =3D the maximum of virtual sizes calculated using [BI= P141 >> virtual size][2] and sigop weight. [Implemented here in Bitcoin >> Core][3]. >> - Note that feerate is not necessarily based on the base fees and >> serialized >> size. >> >> * Fee-Bumping =3D user/wallet actions that take advantage of miner >> incentives to >> boost a transaction's candidacy for inclusion in a block, including >> Child Pays >> for Parent (CPFP) and [BIP125][12] Replace-by-Fee (RBF). Our intention i= n >> mempool policy is to recognize when the new transaction is more >> economical to >> mine than the original one(s) but not open DoS vectors, so there are som= e >> limitations. >> >> ### Policy >> >> The purpose of the mempool is to store the best (to be most >> incentive-compatible >> with miners, highest feerate) candidates for inclusion in a block. Miner= s >> use >> the mempool to build block templates. The mempool is also useful as a >> cache for >> boosting block relay and validation performance, aiding transaction >> relay, and >> generating feerate estimations. >> >> Ideally, all consensus-valid transactions paying reasonable fees should >> make it >> to miners through normal transaction relay, without any special >> connectivity or >> relationships with miners. On the other hand, nodes do not have unlimite= d >> resources, and a P2P network designed to let any honest node broadcast >> their >> transactions also exposes the transaction validation engine to DoS >> attacks from >> malicious peers. >> >> As such, for unconfirmed transactions we are considering for our mempool= , >> we >> apply a set of validation rules in addition to consensus, primarily to >> protect >> us from resource exhaustion and aid our efforts to keep the highest fee >> transactions. We call this mempool _policy_: a set of (configurable, >> node-specific) rules that transactions must abide by in order to be >> accepted >> into our mempool. Transaction "Standardness" rules and mempool >> restrictions such >> as "too-long-mempool-chain" are both examples of policy. >> >> ### Package Relay and Package Mempool Accept >> >> In transaction relay, we currently consider transactions one at a time f= or >> submission to the mempool. This creates a limitation in the node's >> ability to >> determine which transactions have the highest feerates, since we cannot >> take >> into account descendants (i.e. cannot use CPFP) until all the >> transactions are >> in the mempool. Similarly, we cannot use a transaction's descendants whe= n >> considering it for RBF. When an individual transaction does not meet the >> mempool >> minimum feerate and the user isn't able to create a replacement >> transaction >> directly, it will not be accepted by mempools. >> >> This limitation presents a security issue for applications and users >> relying on >> time-sensitive transactions. For example, Lightning and other protocols >> create >> UTXOs with multiple spending paths, where one counterparty's spending >> path opens >> up after a timelock, and users are protected from cheating scenarios as >> long as >> they redeem on-chain in time. A key security assumption is that all >> parties' >> transactions will propagate and confirm in a timely manner. This >> assumption can >> be broken if fee-bumping does not work as intended. >> >> The end goal for Package Relay is to consider multiple transactions at >> the same >> time, e.g. a transaction with its high-fee child. This may help us bette= r >> determine whether transactions should be accepted to our mempool, >> especially if >> they don't meet fee requirements individually or are better RBF >> candidates as a >> package. A combination of changes to mempool validation logic, policy, a= nd >> transaction relay allows us to better propagate the transactions with th= e >> highest package feerates to miners, and makes fee-bumping tools more >> powerful >> for users. >> >> The "relay" part of Package Relay suggests P2P messaging changes, but a >> large >> part of the changes are in the mempool's package validation logic. We >> call this >> *Package Mempool Accept*. >> >> ### Previous Work >> >> * Given that mempool validation is DoS-sensitive and complex, it would b= e >> dangerous to haphazardly tack on package validation logic. Many effort= s >> have >> been made to make mempool validation less opaque (see [#16400][4], >> [#21062][5], >> [#22675][6], [#22796][7]). >> * [#20833][8] Added basic capabilities for package validation, test >> accepts only >> (no submission to mempool). >> * [#21800][9] Implemented package ancestor/descendant limit checks for >> arbitrary >> packages. Still test accepts only. >> * Previous package relay proposals (see [#16401][10], [#19621][11]). >> >> ### Existing Package Rules >> >> These are in master as introduced in [#20833][8] and [#21800][9]. I'll >> consider >> them as "given" in the rest of this document, though they can be changed= , >> since >> package validation is test-accept only right now. >> >> 1. A package cannot exceed `MAX_PACKAGE_COUNT=3D25` count and >> `MAX_PACKAGE_SIZE=3D101KvB` total size [8] >> >> *Rationale*: This is already enforced as mempool ancestor/descendant >> limits. >> Presumably, transactions in a package are all related, so exceeding this >> limit >> would mean that the package can either be split up or it wouldn't pass >> this >> mempool policy. >> >> 2. Packages must be topologically sorted: if any dependencies exist >> between >> transactions, parents must appear somewhere before children. [8] >> >> 3. A package cannot have conflicting transactions, i.e. none of them can >> spend >> the same inputs. This also means there cannot be duplicate transactions. >> [8] >> >> 4. When packages are evaluated against ancestor/descendant limits in a >> test >> accept, the union of all of their descendants and ancestors is >> considered. This >> is essentially a "worst case" heuristic where every transaction in the >> package >> is treated as each other's ancestor and descendant. [8] >> Packages for which ancestor/descendant limits are accurately captured by >> this >> heuristic: [19] >> >> There are also limitations such as the fact that CPFP carve out is not >> applied >> to package transactions. #20833 also disables RBF in package validation; >> this >> proposal overrides that to allow packages to use RBF. >> >> ## Proposed Changes >> >> The next step in the Package Mempool Accept project is to implement >> submission >> to mempool, initially through RPC only. This allows us to test the >> submission >> logic before exposing it on P2P. >> >> ### Summary >> >> - Packages may contain already-in-mempool transactions. >> - Packages are 2 generations, Multi-Parent-1-Child. >> - Fee-related checks use the package feerate. This means that wallets ca= n >> create a package that utilizes CPFP. >> - Parents are allowed to RBF mempool transactions with a set of rules >> similar >> to BIP125. This enables a combination of CPFP and RBF, where a >> transaction's descendant fees pay for replacing mempool conflicts. >> >> There is a draft implementation in [#22290][1]. It is WIP, but feedback = is >> always welcome. >> >> ### Details >> >> #### Packages May Contain Already-in-Mempool Transactions >> >> A package may contain transactions that are already in the mempool. We >> remove >> ("deduplicate") those transactions from the package for the purposes of >> package >> mempool acceptance. If a package is empty after deduplication, we do >> nothing. >> >> *Rationale*: Mempools vary across the network. It's possible for a paren= t >> to be >> accepted to the mempool of a peer on its own due to differences in polic= y >> and >> fee market fluctuations. We should not reject or penalize the entire >> package for >> an individual transaction as that could be a censorship vector. >> >> #### Packages Are Multi-Parent-1-Child >> >> Only packages of a specific topology are permitted. Namely, a package is >> exactly >> 1 child with all of its unconfirmed parents. After deduplication, the >> package >> may be exactly the same, empty, 1 child, 1 child with just some of its >> unconfirmed parents, etc. Note that it's possible for the parents to be >> indirect >> descendants/ancestors of one another, or for parent and child to share a >> parent, >> so we cannot make any other topology assumptions. >> >> *Rationale*: This allows for fee-bumping by CPFP. Allowing multiple >> parents >> makes it possible to fee-bump a batch of transactions. Restricting >> packages to a >> defined topology is also easier to reason about and simplifies the >> validation >> logic greatly. Multi-parent-1-child allows us to think of the package as >> one big >> transaction, where: >> >> - Inputs =3D all the inputs of parents + inputs of the child that come f= rom >> confirmed UTXOs >> - Outputs =3D all the outputs of the child + all outputs of the parents = that >> aren't spent by other transactions in the package >> >> Examples of packages that follow this rule (variations of example A show >> some >> possibilities after deduplication): ![image][15] >> >> #### Fee-Related Checks Use Package Feerate >> >> Package Feerate =3D the total modified fees divided by the total virtual >> size of >> all transactions in the package. >> >> To meet the two feerate requirements of a mempool, i.e., the >> pre-configured >> minimum relay feerate (`minRelayTxFee`) and dynamic mempool minimum >> feerate, the >> total package feerate is used instead of the individual feerate. The >> individual >> transactions are allowed to be below feerate requirements if the package >> meets >> the feerate requirements. For example, the parent(s) in the package can >> have 0 >> fees but be paid for by the child. >> >> *Rationale*: This can be thought of as "CPFP within a package," solving >> the >> issue of a parent not meeting minimum fees on its own. This allows L2 >> applications to adjust their fees at broadcast time instead of >> overshooting or >> risking getting stuck/pinned. >> >> We use the package feerate of the package *after deduplication*. >> >> *Rationale*: It would be incorrect to use the fees of transactions that >> are >> already in the mempool, as we do not want a transaction's fees to be >> double-counted for both its individual RBF and package RBF. >> >> Examples F and G [14] show the same package, but P1 is submitted >> individually before >> the package in example G. In example F, we can see that the 300vB packag= e >> pays >> an additional 200sat in fees, which is not enough to pay for its own >> bandwidth >> (BIP125#4). In example G, we can see that P1 pays enough to replace M1, >> but >> using P1's fees again during package submission would make it look like = a >> 300sat >> increase for a 200vB package. Even including its fees and size would not >> be >> sufficient in this example, since the 300sat looks like enough for the >> 300vB >> package. The calculcation after deduplication is 100sat increase for a >> package >> of size 200vB, which correctly fails BIP125#4. Assume all transactions >> have a >> size of 100vB. >> >> #### Package RBF >> >> If a package meets feerate requirements as a package, the parents in the >> transaction are allowed to replace-by-fee mempool transactions. The chil= d >> cannot >> replace mempool transactions. Multiple transactions can replace the same >> transaction, but in order to be valid, none of the transactions can try = to >> replace an ancestor of another transaction in the same package (which >> would thus >> make its inputs unavailable). >> >> *Rationale*: Even if we are using package feerate, a package will not >> propagate >> as intended if RBF still requires each individual transaction to meet th= e >> feerate requirements. >> >> We use a set of rules slightly modified from BIP125 as follows: >> >> ##### Signaling (Rule #1) >> >> All mempool transactions to be replaced must signal replaceability. >> >> *Rationale*: Package RBF signaling logic should be the same for package >> RBF and >> single transaction acceptance. This would be updated if single transacti= on >> validation moves to full RBF. >> >> ##### New Unconfirmed Inputs (Rule #2) >> >> A package may include new unconfirmed inputs, but the ancestor feerate o= f >> the >> child must be at least as high as the ancestor feerates of every >> transaction >> being replaced. This is contrary to BIP125#2, which states "The >> replacement >> transaction may only include an unconfirmed input if that input was >> included in >> one of the original transactions. (An unconfirmed input spends an output >> from a >> currently-unconfirmed transaction.)" >> >> *Rationale*: The purpose of BIP125#2 is to ensure that the replacement >> transaction has a higher ancestor score than the original transaction(s) >> (see >> [comment][13]). Example H [16] shows how adding a new unconfirmed input >> can lower the >> ancestor score of the replacement transaction. P1 is trying to replace >> M1, and >> spends an unconfirmed output of M2. P1 pays 800sat, M1 pays 600sat, and >> M2 pays >> 100sat. Assume all transactions have a size of 100vB. While, in >> isolation, P1 >> looks like a better mining candidate than M1, it must be mined with M2, >> so its >> ancestor feerate is actually 4.5sat/vB. This is lower than M1's ancesto= r >> feerate, which is 6sat/vB. >> >> In package RBF, the rule analogous to BIP125#2 would be "none of the >> transactions in the package can spend new unconfirmed inputs." Example J >> [17] shows >> why, if any of the package transactions have ancestors, package feerate >> is no >> longer accurate. Even though M2 and M3 are not ancestors of P1 (which is >> the >> replacement transaction in an RBF), we're actually interested in the >> entire >> package. A miner should mine M1 which is 5sat/vB instead of M2, M3, P1, >> P2, and >> P3, which is only 4sat/vB. The Package RBF rule cannot be loosened to >> only allow >> the child to have new unconfirmed inputs, either, because it can still >> cause us >> to overestimate the package's ancestor score. >> >> However, enforcing a rule analogous to BIP125#2 would not only make >> Package RBF >> less useful, but would also break Package RBF for packages with parents >> already >> in the mempool: if a package parent has already been submitted, it would >> look >> like the child is spending a "new" unconfirmed input. In example K [18], >> we're >> looking to replace M1 with the entire package including P1, P2, and P3. >> We must >> consider the case where one of the parents is already in the mempool (in >> this >> case, P2), which means we must allow P3 to have new unconfirmed inputs. >> However, >> M2 lowers the ancestor score of P3 to 4.3sat/vB, so we should not replac= e >> M1 >> with this package. >> >> Thus, the package RBF rule regarding new unconfirmed inputs is less >> strict than >> BIP125#2. However, we still achieve the same goal of requiring the >> replacement >> transactions to have a ancestor score at least as high as the original >> ones. As >> a result, the entire package is required to be a higher feerate mining >> candidate >> than each of the replaced transactions. >> >> Another note: the [comment][13] above the BIP125#2 code in the original >> RBF >> implementation suggests that the rule was intended to be temporary. >> >> ##### Absolute Fee (Rule #3) >> >> The package must increase the absolute fee of the mempool, i.e. the tota= l >> fees >> of the package must be higher than the absolute fees of the mempool >> transactions >> it replaces. Combined with the CPFP rule above, this differs from BIP125 >> Rule #3 >> - an individual transaction in the package may have lower fees than the >> transaction(s) it is replacing. In fact, it may have 0 fees, and the >> child >> pays for RBF. >> >> ##### Feerate (Rule #4) >> >> The package must pay for its own bandwidth; the package feerate must be >> higher >> than the replaced transactions by at least minimum relay feerate >> (`incrementalRelayFee`). Combined with the CPFP rule above, this differs >> from >> BIP125 Rule #4 - an individual transaction in the package can have a low= er >> feerate than the transaction(s) it is replacing. In fact, it may have 0 >> fees, >> and the child pays for RBF. >> >> ##### Total Number of Replaced Transactions (Rule #5) >> >> The package cannot replace more than 100 mempool transactions. This is >> identical >> to BIP125 Rule #5. >> >> ### Expected FAQs >> >> 1. Is it possible for only some of the package to make it into the >> mempool? >> >> Yes, it is. However, since we evict transactions from the mempool by >> descendant score and the package child is supposed to be sponsoring the >> fees of >> its parents, the most common scenario would be all-or-nothing. This is >> incentive-compatible. In fact, to be conservative, package validation >> should >> begin by trying to submit all of the transactions individually, and only >> use the >> package mempool acceptance logic if the parents fail due to low feerate. >> >> 2. Should we allow packages to contain already-confirmed transactions? >> >> No, for practical reasons. In mempool validation, we actually aren't >> able to >> tell with 100% confidence if we are looking at a transaction that has >> already >> confirmed, because we look up inputs using a UTXO set. If we have >> historical >> block data, it's possible to look for it, but this is inefficient, not >> always >> possible for pruning nodes, and unnecessary because we're not going to d= o >> anything with the transaction anyway. As such, we already have the >> expectation >> that transaction relay is somewhat "stateful" i.e. nobody should be >> relaying >> transactions that have already been confirmed. Similarly, we shouldn't b= e >> relaying packages that contain already-confirmed transactions. >> >> [1]: https://github.com/bitcoin/bitcoin/pull/22290 >> [2]: >> https://github.com/bitcoin/bips/blob/1f0b563738199ca60d32b4ba779797fc97d= 040fe/bip-0141.mediawiki#transaction-size-calculations >> [3]: >> https://github.com/bitcoin/bitcoin/blob/94f83534e4b771944af7d9ed0f40746f= 392eb75e/src/policy/policy.cpp#L282 >> [4]: https://github.com/bitcoin/bitcoin/pull/16400 >> [5]: https://github.com/bitcoin/bitcoin/pull/21062 >> [6]: https://github.com/bitcoin/bitcoin/pull/22675 >> [7]: https://github.com/bitcoin/bitcoin/pull/22796 >> [8]: https://github.com/bitcoin/bitcoin/pull/20833 >> [9]: https://github.com/bitcoin/bitcoin/pull/21800 >> [10]: https://github.com/bitcoin/bitcoin/pull/16401 >> [11]: https://github.com/bitcoin/bitcoin/pull/19621 >> [12]: https://github.com/bitcoin/bips/blob/master/bip-0125.mediawiki >> [13]: >> https://github.com/bitcoin/bitcoin/pull/6871/files#diff-34d21af3c614ea3c= ee120df276c9c4ae95053830d7f1d3deaf009a4625409ad2R1101-R1104 >> [14]: >> https://user-images.githubusercontent.com/25183001/133567078-075a971c-06= 19-4339-9168-b41fd2b90c28.png >> [15]: >> https://user-images.githubusercontent.com/25183001/132856734-fc17da75-f8= 75-44bb-b954-cb7a1725cc0d.png >> [16]: >> https://user-images.githubusercontent.com/25183001/133567347-a3e2e4a8-ae= 9c-49f8-abb9-81e8e0aba224.png >> [17]: >> https://user-images.githubusercontent.com/25183001/133567370-21566d0e-36= c8-4831-b1a8-706634540af3.png >> [18]: >> https://user-images.githubusercontent.com/25183001/133567444-bfff1142-43= 9f-4547-800a-2ba2b0242bcb.png >> [19]: >> https://user-images.githubusercontent.com/25183001/133456219-0bb447cb-dc= b4-4a31-b9c1-7d86205b68bc.png >> [20]: >> https://user-images.githubusercontent.com/25183001/132857787-7b7c6f56-af= 96-44c8-8d78-983719888c19.png >> _______________________________________________ >> bitcoin-dev mailing list >> bitcoin-dev@lists.linuxfoundation.org >> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >> > --0000000000003984f005cc6eae1f Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Hi Antoine,

First of all, thank you for the tho= rough review. I appreciate your insight on LN requirements.

> IIU= C, you have a package A+B+C submitted for acceptance and A is already in yo= ur mempool. You trim out A from the package and then evaluate B+C.

&= gt; I think this might be an issue if A is the higher-fee element of the AB= C package. B+C package fees might be under the mempool min fee and will be = rejected, potentially breaking the acceptance expectations of the package i= ssuer ?

Correct, if B+C is too low feerate to be accepted, we will r= eject it. I prefer this because it is incentive compatible: A can be mined = by itself, so there's no reason to prefer A+B+C instead of A.
As ano= ther way of looking at this, consider the case where we do accept A+B+C and= it sits at the "bottom" of our mempool. If our mempool reaches c= apacity, we evict the lowest descendant feerate transactions, which are B+C= in this case. This gives us the same resulting mempool, with A and not B+C= .


> Further, I think the dedup should b= e done on wtxid, as you might have multiple valid witnesses. Though with va= rying vsizes and as such offering different feerates.

I agree that v= ariations of the same package with different witnesses is a case that must = be handled. I consider witness replacement to be a project that can be done= in parallel to package mempool acceptance because being able to accept pac= kages does not worsen the problem of a same-txid-different-witness "pi= nning" attack.

If or when we have witness replacement, the logi= c is: if the individual transaction is enough to replace the mempool one, t= he replacement will happen during the preceding individual transaction acce= ptance, and deduplication logic will work. Otherwise, we will try to dedupl= icate by wtxid, see that we need a package witness replacement, and use the= package feerate to evaluate whether this is economically rational.

= See the #22290 "handle package transactions already in mempool" c= ommit (https://github.com/bitcoin/bitcoin/p= ull/22290/commits/fea75a2237b46cf76145242fecad7e274bfcb5ff), which hand= les the case of same-txid-different-witness by simply using the transaction= in the mempool for now, with TODOs for what I just described.


&= gt; I'm not clearly understanding the accepted topologies. By "par= ent and child to share a parent", do you mean the set of transactions = A, B, C, where B is spending A and C is spending A and B would be correct ?=

Yes, that is what I meant. Yes, that would a valid package under th= ese rules.

> If yes, is there a width-limit introduced or we fall= back on MAX_PACKAGE_COUNT=3D25 ?

No, there is no limit on connectivi= ty other than "child with all unconfirmed parents." We will enfor= ce MAX_PACKAGE_COUNT=3D25 and child's in-mempool + in-package ancestor = limits.


> Considering the current Core's mempool acceptan= ce rules, I think CPFP batching is unsafe for LN time-sensitive closure. A = malicious tx-relay jamming successful on one channel commitment transaction= would contamine the remaining commitments sharing the same package.
> E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitment= transactions and E a shared CPFP. If a malicious A' transaction has a = better feerate than A, the whole package acceptance will fail. Even if A= 9; confirms in the following block,
the propagation and confirmation of = B+C+D have been delayed. This could carry on a loss of funds.

Please= note that A may replace A' even if A' has higher fees than A indiv= idually, because the proposed package RBF utilizes the fees and size of the= entire package. This just requires E to pay enough fees, although this can= be pretty high if there are also potential B' and C' competing com= mitment transactions that we don't know about.


> IMHO, I&= #39;m leaning towards deploying during a first phase 1-parent/1-child. I th= ink it's the most conservative step still improving second-layer safety= .

So far, my understanding is that multi-parent-1-child is desired f= or batched fee-bumping (https://github.com/bitcoin/bitcoin/pull/22674= #issuecomment-897951289) and I've also seen your response which I h= ave less context on (https://github.com/bitcoin/bitcoin/pull/22674#is= suecomment-900352202). That being said, I am happy to create a new prop= osal for 1 parent + 1 child (which would be slightly simpler) and plan for = moving to multi-parent-1-child later if that is preferred. I am very intere= sted in hearing feedback on that approach.


> If A+B is submit= ted to replace A', where A pays 0 sats, B pays 200 sats and A' pays= 100 sats. If we apply the individual RBF on A, A+B acceptance fails. For t= his reason I think the individual RBF should be bypassed and only the packa= ge RBF apply ?

I think there is a misunderstanding here - let me des= cribe what I'm proposing we'd do in this situation: we'll try i= ndividual submission for A, see that it fails due to "insufficient fee= s." Then, we'll try package validation for A+B and use package RBF= . If A+B pays enough, it can still replace A'. If A fails for a bad sig= nature, we won't look at B or A+B. Does this meet your expectations?

> What problem are you trying to solve by the package feerate *= after* dedup rule ?
> My understanding is that an in-package transact= ion might be already in the mempool. Therefore, to compute a correct RBF pe= nalty replacement, the vsize of this transaction could be discarded lowerin= g the cost of package RBF.

I'm proposing that, when a transactio= n has already been submitted to mempool, we would ignore both its fees and = vsize when calculating package feerate. In example G2, we shouldn't cou= nt M1 fees after its submission to mempool, since M1's fees have alread= y been used to pay for its individual bandwidth, and it shouldn't be us= ed again to pay for P2 and P3's bandwidth. We also shouldn't count = its vsize, since it has already been paid for.


> I think this= is a footgunish API, as if a package issuer send the multiple-parent-one-c= hild package A,B,C,D where D is the child of A,B,C. Then try to broadcast t= he higher-feerate C'+D' package, it should be rejected. So it's= breaking the naive broadcaster assumption that a higher-feerate/higher-fee= package always replaces ?

Note that, if C' conflicts with = C, it also conflicts with D, since D is a descendant of C and would thus ne= ed to be evicted along with it. Implicitly, D' would not be in conflict= with D.
More generally, this example is surprising to me bec= ause I didn't think packages would be used to fee-bump replaceable tran= sactions. Do we want the child to be able to replace mempool transactions a= s well? This can be implemented with a bit of additional logic.

&g= t; I think this is unsafe for L2s if counterparties have malleability of th= e child transaction. They can block your package replacement by opting-out = from RBF signaling. IIRC, LN's "anchor output" presents such = an ability.

I'm not sure what you mean? Let's say we have a = package of parent A + child B, where A is supposed to replace a mempool tra= nsaction A'. Are you saying that counterparties are able to malleate th= e package child B, or a child of A'? If they can malleate a child of A&= #39;, that shouldn't matter as long as A' is signaling replacement.= This would be handled identically with full RBF and what Core currently im= plements.

> I think this is an issue brought by the trimming duri= ng the dedup phase. If we preserve the package integrity, only re-using the= tx-level checks results of already in-mempool transactions to gain in CPU = time we won't have this issue. Package childs can add unconfirmed input= s as long as they're in-package, the bip125 rule2 is only evaluated aga= inst parents ?

Sorry, I don't understand what you mean by "= preserve the package integrity?" Could you elaborate?

> Let&= #39;s say you have in-mempool A, B where A pays 10 sat/vb for 100 vbytes an= d B pays 10 sat/vb for 100 vbytes. You have the candidate replacement D spe= nding both A and C where D pays 15sat/vb for 100 vbytes and C pays 1 sat/vb= for 1000 vbytes.

> Package A + B ancestor score is 10 sat/vb.
> D has a higher feerate/absolute fee than B.

> Package A= + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats + C's 1000 sa= ts + D's 1500 sats) / A's 100 vb + C's 1000 vb + D's 100 vb= )

I am in agreement with your calculations but unsure if we dis= agree on the expected outcome. Yes, B has an ancestor score of 10sat/vb and= D has an ancestor score of ~2.9sat/vb. Since D's ancestor score is low= er than B's, it fails the proposed package RBF Rule #2, so this package= would be rejected. Does this meet your expectations?

<= div>Thank you for linking to projects that might be interested in package r= elay :)

Thanks,
Gloria

On Mon, Sep 20, 2021 at 12:16 AM = Antoine Riard <antoine.riard@= gmail.com> wrote:
Hi Gloria,

> A package may contain tr= ansactions that are already in the mempool. We
> remove
> (&quo= t;deduplicate") those transactions from the package for the purposes o= f
> package
> mempool acceptance. If a package is empty after d= eduplication, we do
> nothing.

IIUC, you have a package A+B+C = submitted for acceptance and A is already in your mempool. You trim out A f= rom the package and then evaluate B+C.

I think this might be an issu= e if A is the higher-fee element of the ABC package. B+C package fees might= be under the mempool min fee and will be rejected, potentially breaking th= e acceptance expectations of the package issuer ?

Further, I think t= he dedup should be done on wtxid, as you might have multiple valid witnesse= s. Though with varying vsizes and as such offering different feerates.
<= br>E.g you're going to evaluate the package A+B and A' is already i= n your mempool with a bigger valid witness. You trim A based on txid, then = you evaluate A'+B, which fails the fee checks. However, evaluating A+B = would have been a success.

AFAICT, the dedup rationale would be to s= ave on CPU time/IO disk, to avoid repeated signatures verification and pare= nt UTXOs fetches ? Can we achieve the same goal by bypassing tx-level check= s for already-in txn while conserving the package integrity for package-lev= el checks ?

> Note that it's possible for the parents to be> indirect
> descendants/ancestors of one another, or for parent= and child to share a
> parent,
> so we cannot make any other t= opology assumptions.

I'm not clearly understanding the accepted = topologies. By "parent and child to share a parent", do you mean = the set of transactions A, B, C, where B is spending A and C is spending A = and B would be correct ?

If yes, is there a width-limit introduced o= r we fallback on MAX_PACKAGE_COUNT=3D25 ?

IIRC, one rationale to com= e with this topology limitation was to lower the DoS risks when potentially= deploying p2p packages.

Considering the current Core's mempool = acceptance rules, I think CPFP batching is unsafe for LN time-sensitive clo= sure. A malicious tx-relay jamming successful on one channel commitment tra= nsaction would contamine the remaining commitments sharing the same package= .

E.g, you broadcast the package A+B+C+D+E where A,B,C,D are commitm= ent transactions and E a shared CPFP. If a malicious A' transaction has= a better feerate than A, the whole package acceptance will fail. Even if A= ' confirms in the following block,
the propagation and confirmation= of B+C+D have been delayed. This could carry on a loss of funds.

Th= at said, if you're broadcasting commitment transactions without time-se= nsitive HTLC outputs, I think the batching is effectively a fee saving as y= ou don't have to duplicate the CPFP.

IMHO, I'm leaning towar= ds deploying during a first phase 1-parent/1-child. I think it's the mo= st conservative step still improving second-layer safety.

> *Rati= onale*: =C2=A0It would be incorrect to use the fees of transactions that ar= e
> already in the mempool, as we do not want a transaction's fee= s to be
> double-counted for both its individual RBF and package RBF.=

I'm unsure about the logical order of the checks proposed.
<= br>If A+B is submitted to replace A', where A pays 0 sats, B pays 200 s= ats and A' pays 100 sats. If we apply the individual RBF on A, A+B acce= ptance fails. For this reason I think the individual RBF should be bypassed= and only the package RBF apply ?

Note this situation is plausible,= with current LN design, your counterparty can have a commitment transactio= n with a better fee just by selecting a higher `dust_limit_satoshis` than y= ours.

> Examples F and G [14] show the same package, but P1 is su= bmitted
> individually before
> the package in example G. In ex= ample F, we can see that the 300vB package
> pays
> an addition= al 200sat in fees, which is not enough to pay for its own
> bandwidth=
> (BIP125#4). In example G, we can see that P1 pays enough to replac= e M1, but
> using P1's fees again during package submission would= make it look like a
> 300sat
> increase for a 200vB package. E= ven including its fees and size would not be
> sufficient in this exa= mple, since the 300sat looks like enough for the 300vB
> package. The= calculcation after deduplication is 100sat increase for a
> package<= br>> of size 200vB, which correctly fails BIP125#4. Assume all transacti= ons have
> a
> size of 100vB.

What problem are you tryin= g to solve by the package feerate *after* dedup rule ?

My understand= ing is that an in-package transaction might be already in the mempool. Ther= efore, to compute a correct RBF penalty replacement, the vsize of this tran= saction could be discarded lowering the cost of package RBF.

If we k= eep a "safe" dedup mechanism (see my point above), I think this d= iscount is justified, as the validation cost of node operators is paid for = ?

> The child cannot replace mempool transactions.

Let'= ;s say you issue package A+B, then package C+B', where B' is a chil= d of both A and C. This rule fails the acceptance of C+B' ?

I th= ink this is a footgunish API, as if a package issuer send the multiple-pare= nt-one-child package A,B,C,D where D is the child of A,B,C. Then try to bro= adcast the higher-feerate C'+D' package, it should be rejected. So = it's breaking the naive broadcaster assumption that a higher-feerate/hi= gher-fee package always replaces ? And it might be unsafe in protocols wher= e states are symmetric. E.g a malicious counterparty broadcasts first S+A, = then you honestly broadcast S+B, where B pays better fees.

> All = mempool transactions to be replaced must signal replaceability.

I th= ink this is unsafe for L2s if counterparties have malleability of the child= transaction. They can block your package replacement by opting-out from RB= F signaling. IIRC, LN's "anchor output" presents such an abil= ity.

I think it's better to either fix inherited signaling or mo= ve towards full-rbf.

> if a package parent has already been submi= tted, it would
> look
>like the child is spending a "new&q= uot; unconfirmed input.

I think this is an issue brought by the trim= ming during the dedup phase. If we preserve the package integrity, only re-= using the tx-level checks results of already in-mempool transactions to gai= n in CPU time we won't have this issue. Package childs can add unconfir= med inputs as long as they're in-package, the bip125 rule2 is only eval= uated against parents ?

> However, we still achieve the same goal= of requiring the
> replacement
> transactions to have a ancest= or score at least as high as the original
> ones.

I'm not = sure if this holds...

Let's say you have in-mempool A, B where A= pays 10 sat/vb for 100 vbytes and B pays 10 sat/vb for 100 vbytes. You hav= e the candidate replacement D spending both A and C where D pays 15sat/vb f= or 100 vbytes and C pays 1 sat/vb for 1000 vbytes.

Package A + B anc= estor score is 10 sat/vb.

D has a higher feerate/absolute fee than B= .

Package A + C + D ancestor score is ~ 3 sat/vb ((A's 1000 sats= + C's 1000 sats + D's 1500 sats) /
A's 100 vb + C's 10= 00 vb + D's 100 vb)

Overall, this is a review through the lenses= of LN requirements. I think other L2 protocols/applications
could be ca= ndidates to using package accept/relay such as:
* https://github.com/lightningl= abs/pool
* https://github.com/discreetlogcontracts/dlcspecs<= br>* https://github.com/bitcoin-teleport/teleport-transaction= s/
* https://github.com/sapio-lang/sapio
* = https://github.com/commerceblock/mercury/blob/master/doc/statechains.md=
* https://github.com/revault/practical-revault

Thanks for ro= lling forward the ball on this subject.

Antoine

Le=C2=A0jeu. 16 s= ept. 2021 =C3=A0=C2=A003:55, Gloria Zhao via bitcoin-dev <bitcoin-dev@li= sts.linuxfoundation.org> a =C3=A9crit=C2=A0:
Hi there,

I'= ;m writing to propose a set of mempool policy changes to enable package
= validation (in preparation for package relay) in Bitcoin Core. These would = not
be consensus or P2P protocol changes. However, since mempool policy<= br>significantly affects transaction propagation, I believe this is relevan= t for
the mailing list.

My proposal enables packages consisting o= f multiple parents and 1 child. If you
develop software that relies on s= pecific transaction relay assumptions and/or
are interested in using pac= kage relay in the future, I'm very interested to hear
your feedback = on the utility or restrictiveness of these package policies for
your use= cases.

A draft implementation of this proposal can be found in [Bit= coin Core
PR#22290][1].

An illustrated version of this post can b= e found at
I have also linked the images bel= ow.

## Background

Feel free to skip this section if you are= already familiar with mempool policy
and package relay terminology.
=
### Terminology Clarifications

* Package =3D an ordered list of = related transactions, representable by a Directed
=C2=A0 Acyclic Graph.<= br>* Package Feerate =3D the total modified fees divided by the total virtu= al size of
=C2=A0 all transactions in the package.
=C2=A0 =C2=A0 - Mo= dified fees =3D a transaction's base fees + fee delta applied by the us= er
=C2=A0 =C2=A0 =C2=A0 with `prioritisetransaction`. As such, we expect= this to vary across
mempools.
=C2=A0 =C2=A0 - Virtual Size =3D the m= aximum of virtual sizes calculated using [BIP141
=C2=A0 =C2=A0 =C2=A0 vi= rtual size][2] and sigop weight. [Implemented here in Bitcoin Core][3].
= =C2=A0 =C2=A0 - Note that feerate is not necessarily based on the base fees= and serialized
=C2=A0 =C2=A0 =C2=A0 size.

* Fee-Bumping =3D user= /wallet actions that take advantage of miner incentives to
=C2=A0 boost = a transaction's candidacy for inclusion in a block, including Child Pay= s
for Parent (CPFP) and [BIP125][12] Replace-by-Fee (RBF). Our intention= in
mempool policy is to recognize when the new transaction is more econ= omical to
mine than the original one(s) but not open DoS vectors, so the= re are some
limitations.

### Policy

The purpose of the mem= pool is to store the best (to be most incentive-compatible
with miners, = highest feerate) candidates for inclusion in a block. Miners use
the mem= pool to build block templates. The mempool is also useful as a cache forboosting block relay and validation performance, aiding transaction relay,= and
generating feerate estimations.

Ideally, all consensus-valid= transactions paying reasonable fees should make it
to miners through no= rmal transaction relay, without any special connectivity or
relationship= s with miners. On the other hand, nodes do not have unlimited
resources,= and a P2P network designed to let any honest node broadcast their
trans= actions also exposes the transaction validation engine to DoS attacks from<= br>malicious peers.

As such, for unconfirmed transactions we are con= sidering for our mempool, we
apply a set of validation rules in addition= to consensus, primarily to protect
us from resource exhaustion and aid = our efforts to keep the highest fee
transactions. We call this mempool _= policy_: a set of (configurable,
node-specific) rules that transactions = must abide by in order to be accepted
into our mempool. Transaction &quo= t;Standardness" rules and mempool restrictions such
as "too-lo= ng-mempool-chain" are both examples of policy.

### Package Rela= y and Package Mempool Accept

In transaction relay, we currently cons= ider transactions one at a time for
submission to the mempool. This crea= tes a limitation in the node's ability to
determine which transactio= ns have the highest feerates, since we cannot take
into account descenda= nts (i.e. cannot use CPFP) until all the transactions are
in the mempool= . Similarly, we cannot use a transaction's descendants when
consider= ing it for RBF. When an individual transaction does not meet the mempoolminimum feerate and the user isn't able to create a replacement transa= ction
directly, it will not be accepted by mempools.

This limitat= ion presents a security issue for applications and users relying on
time= -sensitive transactions. For example, Lightning and other protocols create<= br>UTXOs with multiple spending paths, where one counterparty's spendin= g path opens
up after a timelock, and users are protected from cheating = scenarios as long as
they redeem on-chain in time. A key security assump= tion is that all parties'
transactions will propagate and confirm in= a timely manner. This assumption can
be broken if fee-bumping does not = work as intended.

The end goal for Package Relay is to consider mult= iple transactions at the same
time, e.g. a transaction with its high-fee= child. This may help us better
determine whether transactions should be= accepted to our mempool, especially if
they don't meet fee requirem= ents individually or are better RBF candidates as a
package. A combinati= on of changes to mempool validation logic, policy, and
transaction relay= allows us to better propagate the transactions with the
highest package= feerates to miners, and makes fee-bumping tools more powerful
for users= .

The "relay" part of Package Relay suggests P2P messaging= changes, but a large
part of the changes are in the mempool's packa= ge validation logic. We call this
*Package Mempool Accept*.

### P= revious Work

* Given that mempool validation is DoS-sensitive and co= mplex, it would be
=C2=A0 dangerous to haphazardly tack on package valid= ation logic. Many efforts have
been made to make mempool validation less= opaque (see [#16400][4], [#21062][5],
[#22675][6], [#22796][7]).
* [= #20833][8] Added basic capabilities for package validation, test accepts on= ly
=C2=A0 (no submission to mempool).
* [#21800][9] Implemented packa= ge ancestor/descendant limit checks for arbitrary
=C2=A0 packages. Still= test accepts only.
* Previous package relay proposals (see [#16401][10]= , [#19621][11]).

### Existing Package Rules

These are in mast= er as introduced in [#20833][8] and [#21800][9]. I'll consider
them = as "given" in the rest of this document, though they can be chang= ed, since
package validation is test-accept only right now.

1. A = package cannot exceed `MAX_PACKAGE_COUNT=3D25` count and
`MAX_PACKAGE_SI= ZE=3D101KvB` total size [8]

=C2=A0 =C2=A0*Rationale*: This is alread= y enforced as mempool ancestor/descendant limits.
Presumably, transactio= ns in a package are all related, so exceeding this limit
would mean that= the package can either be split up or it wouldn't pass this
mempool= policy.

2. Packages must be topologically sorted: if any dependenci= es exist between
transactions, parents must appear somewhere before chil= dren. [8]

3. A package cannot have conflicting transactions, i.e. no= ne of them can spend
the same inputs. This also means there cannot = be duplicate transactions. [8]

4. When packages are eva= luated against ancestor/descendant limits in a test
accept, the union of= all of their descendants and ancestors is considered. This
is essential= ly a "worst case" heuristic where every transaction in the packag= e
is treated as each other's ancestor and descendant. [8]
Packag= es for which ancestor/descendant limits are accurately captured by this
=
heuristic: [19]

There are also limitations such as the fact t= hat CPFP carve out is not applied
to package transactions. #20833 also d= isables RBF in package validation; this
proposal overrides that to allow= packages to use RBF.

## Proposed Changes

The next step in th= e Package Mempool Accept project is to implement submission
to mempool, = initially through RPC only. This allows us to test the submission
logic = before exposing it on P2P.

### Summary

- Packages may contain= already-in-mempool transactions.
- Packages are 2 generations, Multi-Pa= rent-1-Child.
- Fee-related checks use the package feerate. This means t= hat wallets can
create a package that utilizes CPFP.
- Parents are al= lowed to RBF mempool transactions with a set of rules similar
=C2=A0 to = BIP125. This enables a combination of CPFP and RBF, where a
transaction&= #39;s descendant fees pay for replacing mempool conflicts.

There is = a draft implementation in [#22290][1]. It is WIP, but feedback is
always= welcome.

### Details

#### Packages May Contain Already-in-Me= mpool Transactions

A package may contain transactions that are alrea= dy in the mempool. We remove
("deduplicate") those transaction= s from the package for the purposes of package
mempool acceptance. If a = package is empty after deduplication, we do nothing.

*Rationale*: Me= mpools vary across the network. It's possible for a parent to be
acc= epted to the mempool of a peer on its own due to differences in policy and<= br>fee market fluctuations. We should not reject or penalize the entire pac= kage for
an individual transaction as that could be a censorship vector.=

#### Packages Are Multi-Parent-1-Child

Only packages of a sp= ecific topology are permitted. Namely, a package is exactly
1 child with= all of its unconfirmed parents. After deduplication, the package
may be= exactly the same, empty, 1 child, 1 child with just some of its
unconfi= rmed parents, etc. Note that it's possible for the parents to be indire= ct
descendants/ancestors of one another, or for parent and child to shar= e a parent,
so we cannot make any other topology assumptions.

*Ra= tionale*: This allows for fee-bumping by CPFP. Allowing multiple parentsmakes it possible to fee-bump a batch of transactions. Restricting package= s to a
defined topology is also easier to reason about and simplifies th= e validation
logic greatly. Multi-parent-1-child allows us to think of t= he package as one big
transaction, where:

- Inputs =3D all the in= puts of parents + inputs of the child that come from
=C2=A0 confirmed UT= XOs
- Outputs =3D all the outputs of the child + all outputs of the pare= nts that
=C2=A0 aren't spent by other transactions in the package
Examples of packages that follow this rule (variations of example A sh= ow some
possibilities after deduplication): ![image][15]

#### Fee= -Related Checks Use Package Feerate

Package Feerate =3D the total mo= dified fees divided by the total virtual size of
all transactions in the= package.

To meet the two feerate requirements of a mempool, i.e., t= he pre-configured
minimum relay feerate (`minRelayTxFee`) and dynamic me= mpool minimum feerate, the
total package feerate is used instead of the = individual feerate. The individual
transactions are allowed to be below = feerate requirements if the package meets
the feerate requirements. For = example, the parent(s) in the package can have 0
fees but be paid for by= the child.

*Rationale*: This can be thought of as "CPFP within= a package," solving the
issue of a parent not meeting minimum fees= on its own. This allows L2
applications to adjust their fees at broadca= st time instead of overshooting or
risking getting stuck/pinned.

= We use the package feerate of the package *after deduplication*.

*Ra= tionale*: =C2=A0It would be incorrect to use the fees of transactions that = are
already in the mempool, as we do not want a transaction's fees t= o be
double-counted for both its individual RBF and package RBF.

= Examples F and G [14] show the same package, but P1 is submitted individual= ly before
the package in example G. In example F, we can see that the 30= 0vB package pays
an additional 200sat in fees, which is not enough to pa= y for its own bandwidth
(BIP125#4). In example G, we can see that P1 pay= s enough to replace M1, but
using P1's fees again during package sub= mission would make it look like a 300sat
increase for a 200vB package. E= ven including its fees and size would not be
sufficient in this example,= since the 300sat looks like enough for the 300vB
package. The calculcat= ion after deduplication is 100sat increase for a package
of size 200vB, = which correctly fails BIP125#4. Assume all transactions have a
size of 1= 00vB.

#### Package RBF

If a package meets feerate requirement= s as a package, the parents in the
transaction are allowed to replace-by= -fee mempool transactions. The child cannot
replace mempool transactions= . Multiple transactions can replace the same
transaction, but in order t= o be valid, none of the transactions can try to
replace an ancestor of a= nother transaction in the same package (which would thus
make its inputs= unavailable).

*Rationale*: Even if we are using package feerate, a = package will not propagate
as intended if RBF still requires each indivi= dual transaction to meet the
feerate requirements.

We use a set o= f rules slightly modified from BIP125 as follows:

##### Signaling (R= ule #1)

All mempool transactions to be replaced must signal replacea= bility.

*Rationale*: Package RBF signaling logic should be the same = for package RBF and
single transaction acceptance. This would be updated= if single transaction
validation moves to full RBF.

##### New Un= confirmed Inputs (Rule #2)

A package may include new unconfirmed inp= uts, but the ancestor feerate of the
child must be at least as high as t= he ancestor feerates of every transaction
being replaced. This is contra= ry to BIP125#2, which states "The replacement
transaction may only = include an unconfirmed input if that input was included in
one of the or= iginal transactions. (An unconfirmed input spends an output from a
curre= ntly-unconfirmed transaction.)"

*Rationale*: The purpose of BIP= 125#2 is to ensure that the replacement
transaction has a higher ancesto= r score than the original transaction(s) (see
[comment][13]). Example H = [16] shows how adding a new unconfirmed input can lower the
ancestor sco= re of the replacement transaction. P1 is trying to replace M1, and
spend= s an unconfirmed output of M2. P1 pays 800sat, M1 pays 600sat, and M2 pays<= br>100sat. Assume all transactions have a size of 100vB. While, in isolatio= n, P1
looks like a better mining candidate than M1, it must be mined wit= h M2, so its
ancestor feerate is actually 4.5sat/vB.=C2=A0 This is lower= than M1's ancestor
feerate, which is 6sat/vB.

In package RBF= , the rule analogous to BIP125#2 would be "none of the
transactions= in the package can spend new unconfirmed inputs." Example J [17] show= s
why, if any of the package transactions have ancestors, package feerat= e is no
longer accurate. Even though M2 and M3 are not ancestors of P1 (= which is the
replacement transaction in an RBF), we're actually inte= rested in the entire
package. A miner should mine M1 which is 5sat/vB in= stead of M2, M3, P1, P2, and
P3, which is only 4sat/vB. The Package RBF = rule cannot be loosened to only allow
the child to have new unconfirmed = inputs, either, because it can still cause us
to overestimate the packag= e's ancestor score.

However, enforcing a rule analogous to BIP12= 5#2 would not only make Package RBF
less useful, but would also break Pa= ckage RBF for packages with parents already
in the mempool: if a package= parent has already been submitted, it would look
like the child is spen= ding a "new" unconfirmed input. In example K [18], we're
l= ooking to replace M1 with the entire package including P1, P2, and P3. We m= ust
consider the case where one of the parents is already in the mempool= (in this
case, P2), which means we must allow P3 to have new unconfirme= d inputs. However,
M2 lowers the ancestor score of P3 to 4.3sat/vB, so w= e should not replace M1
with this package.

Thus, the package RBF = rule regarding new unconfirmed inputs is less strict than
BIP125#2. Howe= ver, we still achieve the same goal of requiring the replacement
transac= tions to have a ancestor score at least as high as the original ones. Asa result, the entire package is required to be a higher feerate mining can= didate
than each of the replaced transactions.

Another note: the = [comment][13] above the BIP125#2 code in the original RBF
implementation= suggests that the rule was intended to be temporary.

##### Absolute= Fee (Rule #3)

The package must increase the absolute fee of the mem= pool, i.e. the total fees
of the package must be higher than the absolut= e fees of the mempool transactions
it replaces. Combined with the CPFP r= ule above, this differs from BIP125 Rule #3
- an individual transaction = in the package may have lower fees than the
=C2=A0 transaction(s) it is = replacing. In fact, it may have 0 fees, and the child
pays for RBF.
<= br>##### Feerate (Rule #4)

The package must pay for its own bandwidt= h; the package feerate must be higher
than the replaced transactions by = at least minimum relay feerate
(`incrementalRelayFee`). Combined with th= e CPFP rule above, this differs from
BIP125 Rule #4 - an individual tran= saction in the package can have a lower
feerate than the transaction(s) = it is replacing. In fact, it may have 0 fees,
and the child pays for RBF= .

##### Total Number of Replaced Transactions (Rule #5)

The p= ackage cannot replace more than 100 mempool transactions. This is identical=
to BIP125 Rule #5.

### Expected FAQs

1. Is it possible fo= r only some of the package to make it into the mempool?

=C2=A0 =C2= =A0Yes, it is. However, since we evict transactions from the mempool by
= descendant score and the package child is supposed to be sponsoring the fee= s of
its parents, the most common scenario would be all-or-nothing. This= is
incentive-compatible. In fact, to be conservative, package validatio= n should
begin by trying to submit all of the transactions individually,= and only use the
package mempool acceptance logic if the parents fail d= ue to low feerate.

2. Should we allow packages to contain already-co= nfirmed transactions?

=C2=A0 =C2=A0 No, for practical reasons. In me= mpool validation, we actually aren't able to
tell with 100% confiden= ce if we are looking at a transaction that has already
confirmed, becaus= e we look up inputs using a UTXO set. If we have historical
block data, = it's possible to look for it, but this is inefficient, not always
po= ssible for pruning nodes, and unnecessary because we're not going to do=
anything with the transaction anyway. As such, we already have the expe= ctation
that transaction relay is somewhat "stateful" i.e. nob= ody should be relaying
transactions that have already been confirmed. Si= milarly, we shouldn't be
relaying packages that contain already-conf= irmed transactions.

[1]: https://github.com/bitcoin/bitcoin/pull/22= 290
[2]: https://github.com/bitcoin/bips/blob/1f0b563738199ca= 60d32b4ba779797fc97d040fe/bip-0141.mediawiki#transaction-size-calculations<= /a>
[3]: = https://github.com/bitcoin/bitcoin/blob/94f83534e4b771944af7d9ed0f40746f392= eb75e/src/policy/policy.cpp#L282
[4]: https://github.com/bitcoin/bi= tcoin/pull/16400
[5]: https://github.com/bitcoin/bitcoin/pull/21062=
[6]: https://github.com/bitcoin/bitcoin/pull/22675
[7]: ht= tps://github.com/bitcoin/bitcoin/pull/22796
[8]: https://github.com= /bitcoin/bitcoin/pull/20833
[9]: https://github.com/bitcoin/bitcoin= /pull/21800
[10]: https://github.com/bitcoin/bitcoin/pull/16401=
[11]: https://github.com/bitcoin/bitcoin/pull/19621
[12]: https://github.com/bitcoin/bips/blob/master/bip-0125.mediawik= i
[13]: https://github.com/bitcoin/bitcoin/pull/6871/fil= es#diff-34d21af3c614ea3cee120df276c9c4ae95053830d7f1d3deaf009a4625409ad2R11= 01-R1104
[14]: https://user-images.githubusercontent.com/25183001/133567078-075a971c-0= 619-4339-9168-b41fd2b90c28.png
[15]: https://user-images.githubusercontent.com/2518300= 1/132856734-fc17da75-f875-44bb-b954-cb7a1725cc0d.png
[16]: https://user-images.githu= busercontent.com/25183001/133567347-a3e2e4a8-ae9c-49f8-abb9-81e8e0aba224.pn= g
[17]: htt= ps://user-images.githubusercontent.com/25183001/133567370-21566d0e-36c8-483= 1-b1a8-706634540af3.png
[18]: https://user-images.githubusercontent.com/25183001/13356= 7444-bfff1142-439f-4547-800a-2ba2b0242bcb.png
[19]: https://user-images.githubusercont= ent.com/25183001/133456219-0bb447cb-dcb4-4a31-b9c1-7d86205b68bc.png
= [20]: https://user= -images.githubusercontent.com/25183001/132857787-7b7c6f56-af96-44c8-8d78-98= 3719888c19.png
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